The present invention relates to an improvement in vacuum packaging.
In recent years, the blister-packaging technique using metal molds has been widely used. As is well known, the blister-package is feasible by a method in which a sheet or film is at first shaped in a desired form of container by means of a metal mold and, after charging a material or an article to be packed into the cup of the shaped sheet or film and combining with a flat base film or sheet provided with a plurality of small holes for the passage of air, the container is tightly sealed while containing the material or the article therein after replacement of air in the cup-shaped container made by the shaped film and the flat base film or sheet or after evacuation of the container through the holes provided in the flat base film or sheet, the holes being sealed afterwards automatically by the melt sheet. When the close-fit packaging is applied to perishables such as meat, the space inevitably established between the material or the article and the film promotes juice separation, thus spoiling the appearance and causing putrefaction of the packaged material. In order to overcome the above-mentioned disadvantage, there have been proposed several methods including a method in which the size of metal mold is closely coincided with the outer dimensions of the material or article to be packaged, and a method in which the degree of vacuum in the shaped sheet is made so high that the packaging film is forced to be tightly contacted with the material or the article to be packed along its external form. However, the former method is not practical since it is necessary to have a rigorous control of the size of materials or articles to be packed and the insertion of such material or article into the thus formed container is not easy. While, the latter method is disadvantageous in that an excess portion of film is undesirably wrinkled on evacuation and the packaged material or article becomes irregular in shape, thus producing a poor external appearance. In addition, the wrapping film is apt to be broken in the portions folded and wrinkled. Both methods are thus unfavorable from a practical point of view. There is accordingly a strong demand for improvements in the packaging films and methods.
In order to realize a tight package while avoiding the wrinkles occurring in surplus portions of the film, there has been also proposed use of heat shrinkable or elastic films as packaging film. In this connection, however, usual stretched films or elastic films are practically unsatisfactory because of their large residual stress or strain. For example, the stretched film or the elastic film often warps the article or material itself which is packaged therewith or squeezes a soft material or such as ham, sausage or the like, or causes separation at the seal.
As an attempt to give vacuum-tight packages with reduced residual stress, there is disclosed in Japanese Laid-Open Application No. 48-6891 a packaging method utilizing the specific thermal property of film of vinylidene chloride copolymer. In this method, the vinylidene chloride copolymer is extruded in the form of a film and is immediately quenched rapidly to keep the film in an amorphous state, and this film is used for packaging. More precisely, the use of such a film is advantageous in that the film in the amorphous state can be prevented from becoming wrinkled on packaging due to its inherent elasticity and that, after packaging, the wrapped film can be fixed in shape by crystallization thereof and by freezing the strain thereof. In this method, the adverse effect due to the residual stress or the strain as experienced in ordinary shrinkable or elastic films can be avoided.
In this method, however, as it is necessary that the film of vinylidene chloride copolymer formed by extrusion be immediately quenched rapidly to retain amorphous state prior to use. The film-forming step and the packaging step are directly connected so as to complete the packaging procedure within a very short time before crystallization of the rapidly cooled copolymer is induced, and the film is crystallized in a subsequent step such as by heat treatment. Accordingly, this method has the vital disadvantage that if the speeds in related steps, particularly the film-forming speed and the packaging speed, are not synchronized properly, the packaging will become impossible, entailing a loss of materials. Further, owing to the inherent characteristics of the copolymer of vinylidene chloride, the packaging procedure should be carried out at a sufficiently low temperature to avoid crystallization of the film which is remarkably accelerated under high temperature conditions resulting in a loss of elasticity. This poses a serious problem if bacteria remain on the film which is used to wrap raw ham, perishable and the like foods, for which the sterilization at high temperature must be avoided after once packed.